1. Field of the Invention
The present invention relates generally to an image forming method in which an image is formed by forming an electrostatic latent image on a photosensitive member provided with a surface layer containing photosensitive material, supplying toner particles to the electrostatic latent image to form a toner image on the surface of the photosensitive member, while conveying transfer paper to the surface of the above photosensitive member having the toner image formed thereon, bringing the transfer paper into contact with the surface of the photosensitive member at a suitable contact angle and then, transferring the above toner image formed on the surface of the photosensitive member to the transfer paper, and more particularly, to an image forming method using as the above photosensitive member an organic photosensitive member provided with a surface layer containing binder resin and photosensitive material dispersed in the binder resin.
2. Description of the Prior Art
In an image forming apparatus such as an electrophotographic copying machine, a printer or a facsimile, an image forming method has been conventionally adopted in which an image is formed by forming an electrostatic latent image on a photosensitive member provided with a surface layer containing photosensitive material, supplying toner particles to the electrostatic latent image to form a toner image on the surface of the photosensitive member, while conveying transfer paper to the surface of the above photosensitive member having the toner image thus formed thereon, bringing the transfer paper into contact with the surface of the photosensitive member at a suitable contact angle and then, transferring the above toner image formed on the surface of the photosensitive member to the transfer paper.
In this image forming method, a photosensitive member using as photosensitive material in its surface layer inorganic photosensitive material such as selenium, cadmium sulfide or zinc oxide has been generally used.
However, the photosensitive member using selenium, for example, has the disadvantages. For example, the proper conditions for manufacturing are so severe that it is difficult to manufacture the photosensitive member and the manufacturing cost is high. In addition, the selenium photosensitive member has poor resistance to heat and mechanical impact so that handling is required to be careful.
Furthermore, the photosensitive member using zinc oxide and the photosensitive member using cadmium sulfide have the disadvantages. For example, stable sensitivity is not obtained in the humid environment. In addition, when coloring material is added as a sensitizing agent to photosensitive layers of the photosensitive members, the coloring material is degraded in charging the photosensitive members by corona discharge or is discolored in exposing the photosensitive members, thereby to make it impossible to hold stable photosensitivity for a long time.
Therefore, as the result of various studies and developments of the photosensitive members, an organic photosensitive member has been developed in recent years so adapted that photosensitive material having various types of photosensitive material, for example, charge generating material for generating charges by irradiation of light and charge transporting material for transporting the charges generated is dispersed in suitable binder resin to form a surface layer. Such an organic photosensitive member has been widely utilized in an image forming apparatus such as an electrophotographic copying machine because it can be manufactured at low cost and has high sensitivity.
More specifically, the above described organic photosensitive member can easily use a combination of various types of photosensitive material, because various types of photosensitive material can be dispersed in binder resin. The combination of the types of photosensitive material makes it possible to rapidly improve the sensitivity. Further, in preparing this organic photosensitive member, low-cost binder resin can be used. Accordingly, the raw material cost is low. Furthermore, the above photosensitive material is dispersed in a solution obtained by dissolving binder resin in a suitable solvent, and a dispersion thus obtained is applied by a coating method such as spray coating or dip coating, thereby to make it possible to simply form a surface layer having photosensitivity on the surface of the photosensitive member. Accordingly, the photosensitive member can be easily mass-produced, and the manufacturing cost is low.
In such an organic photosensitive member, the surface layer is formed by dispersing the photosensitive material, such as charge transporting material and charge generating material, in the binder resin as described above, so that the hardness of the surface layer on its surface is generally lower than that in the above described photosensitive member using the inorganic photosensitive material due to the softness of the binder resin.
On the other hand, as the above described transfer paper, plain paper containing as loading material an inorganic compound such as clay, talc, titanium white or calcium carbonate in addition to fiber tissue such as cellulose or hemicellulose has been utilized so as to adjust the opacy, whiteness, smoothness, height and the like of the paper.
When this plain paper is conveyed to the organic photosensitive member whose surface layer has a low hardness as described above and is brought into contact with the surface layer of the photosensitive member at a suitable contact angle, the above loading material contained in the plain paper may, in some cases, be embedded in the surface layer of the above photosensitive member.
Particularly in recent years, it has been desired to increase the speed at which an image is formed so that large numbers of copies can be made in a short time with increasing demand for copying. Consequently, the speed of the plain paper which is conveyed to the surface of the photosensitive member is increased and the force exerted when the plain paper is brought into contact with the surface of the above photosensitive member becomes large, so that the above loading material contained in the plain paper is embedded in the surface layer more easily.
Furthermore, regenerated paper has been frequently utilized in recent years in terms of the effective utilization of good resources.
The regenerated paper is generally stiffer than the above plain paper, and contains loading material comprising an inorganic compound such as clay or talc in larger quantities than the plain paper so as to increase the whiteness after regeneration. More specifically, in the above plain paper, the stiffness (L.sup.3 /100) measured by a Clark stiffness tester in the TAPPI standard test of T 451 regulation is approximately 40 to 120, and the ash content representing the content of the above loading material comprising the inorganic compound is 5 to 15% by weight. On the other hand, in the regenerated paper recently used, the above stiffness (L.sup.3 /100) is approximately 180 to 450, and the above ash content is 20 to 40% by weight.
Therefore, hen the above regenerated paper is used as the transfer paper, the above loading material is embedded in larger quantities in the surface layer of the above organic photosensitive member.
If the loading material is thus embedded in &he surface layer of the photosensitive member, light incident on the photosensitive member is interrupted by the loading material embedded. Consequently, the sensitivity of the photosensitive member in a portion where the loading material is embedded is lowered, and an image formed is made irregular in density, for example, to lower the quality of the image formed.